I just measured the voltage of all my 9V adaptors(without a load) and the readings are a little surprising.

The Danelectro ZeroHum is 9.76V, the SoundSense is 9.06V, the Boss ACA-120 is 14.76V! and the Baknor reads a whopping 16.3V!!!

I've already used the Boss ACA-120 and the Baknor adaptors before with various other pedals and never fried anything or noticed anything funny. These voltage readings that I'm getting are without a load attached to the adaptor so I'm guessing that once you put a load on there, the voltage probably drops to appx.9 volts. Would that be correct or are these voltages just too high and maybe I shouldn't use these adaptors at all?

Some adaptors are regulated and filtered and some just have filtering. Unloaded, supplies without regulation will read higher.

I would guess that your Boss tuner would work with any type of adaptor, but if you run it in line, unfiltered supplies may pass on hum.

The hum on the output would be the problem to look for.

It is possible that higher voltages, which may remain high if the tuner is the only load, could be a problem for the tuner. You should measure the voltage on your Boss ACA-120 and Baknor with the tuner plugged in.